Calcium Carbonate‐Mediated Engineering of Polymeric Carbon Nitride Photoanodes for Enhanced Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is regarded as a pivotal technology for sustainable hydrogen production. While polymeric carbon nitride (PCN) has emerged as a promising photoanode candidate, its practical application is frequently hindered by a low specific surface area and inefficient charge separation. Herein, CaCO 3 was utilized as a functional structure‐directing agent in a molten‐salt polymerization process to directly engineer CaCO 3 ‐mediated PCN (CaCN) photoanodes on fluorine‐doped tin oxide (FTO) substrates. The introduced CaCO 3 modulates the polymerization microenvironment, which concurrently promotes the in‐situ growth of flake‐like SnS 2 to construct intimate PCN/SnS 2 heterojunctions, stabilizes cyano‐group defects within the PCN framework, and enlarges the electrochemical active surface area (ECSA). These synergistic effects effectively suppress charge recombination and accelerate interfacial charge transfer, endowing the optimal CaCN with a photocurrent density of 316 µA cm −2 at 1.23 V versus RHE under AM 1.5G illumination, which is 3.3‐fold higher than that of the pristine PCN. This CaCO 3 ‐mediated strategy offers a scalable and environmentally benign route for enhancing PCN‐based photoanodes, providing meaningful guidance for the rational design of high‐performance, metal‐free photoelectrodes toward efficient solar energy conversion.